Viral infections account for significant morbidity and mortality in humans and animals. In addition, viral infections also result in significant agricultural losses, with plant viruses causing an estimated $60 billion in lost crop yields throughout the world each year. Although significant resources have been dedicated to identifying compounds having anti-viral properties, viral infections continue to present a significant risk to human health and agriculture.
In addition, the usefulness of most existing anti-viral treatments is limited by the development of multidrug resistance, poor efficacy, and/or toxicity. In fact, many anti-viral treatments are highly toxic and can cause serious side effects, including heart damage, kidney failure and osteoporosis. Other challenges include creating a drug that is broadly applicable in combating many different types of viral infections, which can be particularly important in the treatment of immunocompromised individuals.
One virus in particular, the human immunodeficiency virus (HIV), remains a global pandemic despite the development of antiretroviral drugs targeting HIV. As of 2007, it was estimated that more than 33 million people were infected with HIV, and HIV associated diseases represent a major world health problem. HIV is a retrovirus that infects CD4+ cells of the immune system, destroying or impairing their function. As the infection progresses, the immune system becomes weaker, leaving the infected person more susceptible to opportunistic infections and tumors, such as Kaposi's sarcoma, cervical cancer, lymphoma, and neurological disorders. The most advanced stage of HIV infection is acquired immunodeficiency syndrome (AIDS). It can take 10-15 years for an HIV-infected person to develop AIDS. Certain antiretroviral drugs can delay the process even further.
Although much effort has been put forth into designing effective therapeutics against HIV, currently no curative anti-retroviral drugs against HIV exist. Several stages of the HIV life cycle have been evaluated as targets for the development of therapeutic agents (Mitsuya, H. et al., 1991, FASEB J 5:2369-2381). One area of focus has been the HIV reverse transcriptase enzyme. Reverse transcriptase copies the HIV, single stranded RNA genome into double-stranded viral DNA. The viral DNA is then integrated into the host's chromosomal DNA where the host's cellular processes, like transcription and translation, are used to produce viral proteins and ultimately new virus particles. Therefore, interfering with reverse transcriptase inhibits HIV's ability to replicate. One class of reverse transcriptase inhibitors is nucleoside analogs, such as Zidovudine (AZT), Didanosine (ddI), Zalcitabine (ddC), and Stavudine (d4T), Lamivudine (3TC), Abacavir (ABC), Emtricitabine (FTC), Entecavir (INN), and Apricitabine (ATC) (Mitsuya, H. et al., 1991, Science 249:1533-1544; El Kouni, Curr Pharm Des, 2002, 8:581-93; Sharma et al., Cur Top Med Chem, 2004, 4:895-919). Another class of reverse transcriptase inhibitors is nucleotide analogs, such as Tenofovir (tenofovir disoproxil fumarate) and Adefovir (bis-POM PMPA) (Palmer et al., AIDS Res Hum Retroviruses, 2001, 17:1167-73). These nucleoside and nucleotide compounds are analogs of the naturally occurring deoxyribose nucleotides, however, the analogs lack the 3′-hydroxyl group on the deoxyribose sugar. As a result, when the analogs are incorporated into a growing viral DNA chain, the incoming deoxynucleotide cannot form a phosphodiester bond with the analog that is needed to extend the DNA chain. Thus, the analogs terminate viral DNA replication. Another class of reverse transcriptase inhibitors is the non-nucleoside reverse transcriptase inhibitors, such as Efavirenz, Nevirapine, Delavirdine, and Etravirine (El Safadi et al., Appl Microbiol Biotechnol, 2007, 75:723-37). They have a different mode of action than the nucleoside and nucleotide inhibitors, binding to the reverse transcriptase and interfering with its function.
The late stages of HIV replication involve processing of certain viral proteins prior to the final assembly of new virions. This late-stage processing is dependent, in part, on the activity of a viral protease. Thus, another area of focus in the development of antiretroviral drugs is protease inhibitors, such as saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir, and atazanavir (Erickson, J., 1990, Science 249:527-533; Klei et al., J Virol, 81:9525-35).
Other antiretroviral drugs target viral entry into the cell, the earliest stage of HIV infection. For HIV to enter a cell, its surface gp120 protein binds to CD4, exposing a conserved region of gp120 that binds to a CCR5 or a CXCR4 co-receptor. After gp120 binds to the co-receptor, a hydrophobic fusion peptide at the N-terminus of the gp41 envelope protein is exposed and inserted into the membrane of the cell. Entry inhibitors work by interfering with any stage of the viral entry process. For example, recombinant soluble CD4, for example, has been shown to inhibit infection of CD-4+ T-cells by some HIV-1 strains (Smith, D. H. et al., 1987, Science 238:1704-1707). Similarly, TNX-355 is a monoclonal antibody that binds CD4 and inhibits binding to gp120 (Kuritzkes et al., J Infect Dis, 2004, 189:286-91). BMS-806 binds to the viral envelope protein and inhibits binding to CD4 (Veazy et al., Nature 2003, 438:99-102). Co-receptor binding can be inhibited by several CCR5 inhibitors, including SCH-C and SCH-D, UK-427,857, maraviroc, vicriviroc, and an anti-CCR5 antibody (PRO-140) (Emmelkamp et al., Eur J Med Res, 2007, 12:409-17). Co-receptor binding can also be inhibited by the CXCR4 inhibitors AMD3100 and AMD070 (De Clerq, Nature Reviews Drug Discovery 2003, 2:581-87). Other compounds, such as enfuvirtide, bind to gp41 and interfere with its ability to mediate membrane fusion and entry (La Bonte et al., Nature Reviews Drug Discovery 2003, 2:345-36).
While beneficial, these antiretroviral drugs often exhibit toxic side effects such as bone marrow suppression, vomiting, and liver function abnormalities. In addition, they are not curative, probably due to the rapid appearance of drug resistant HIV mutants (Lander, B. et al., 1989, Science 243:1731-1734). Drug-resistant HIV strains develop due to the very high genetic variability of HIV. This genetic variability results from several factors, including HIV's fast replication cycle, with the generation of 109 to 1010 virions per day, a high mutation rate of approximately 3×10−5 per nucleotide base per cycle of replication, and recombinogenic properties of reverse transcriptase.
To combat the development of drug resistant HIV strains, multiple drugs have been combined as part of highly active antiretroviral therapy (HAART) (El Safadi et al., Appl Microbiol Biotechnol, 2007, 75:723-37; Sharma et al., Cur Top Med Chem, 2004, 4:895-919). Currently HAART typically involves combining at least three drugs belonging to at least two classes of antiretroviral agents. As discussed above, these classes include nucleoside or nucleotide analog reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors, and entry inhibitors.
Thus, although a great deal of effort is being directed to the design and testing of anti-viral drugs, the search for new and improved methods of treating viral infections, such as HIV, continues.